Compressible plate for flexographic printing and method for obtaining same

ABSTRACT

The invention concerns a compressible plate for flexographic printing, designed to be positioned on a support cylinder of a printing machine. The invention is characterised in that said plate ( 1 ) comprises a photopolymer layer ( 10 ) and a compressible layer ( 11 ) made of a material exhibiting voids forming between 10 and 60% by volume of said compressible layer, the material comprising at least 70% by weight of a polymer or a mixture of polymers chemically compatible with the photopolymer.

The invention relates to a compressible plate for flexographic printingwhich is intended to be positioned on a support cylinder of a printingmachine.

There are known numerous printing plates which comprise a photopolymerlayer placed on a support, which is generally a polyester film. Prior touse of the printing plate, the photopolymer layer is developed toreceive the image to be printed. Such development comprises exposing thephotopolymer layer to UV light through a mask, the unexposed zones ofthe photopolymer layer then being dissolved away by a solvent. As aresult, there is obtained a layer with relief, which will make itpossible to reproduce the image.

After development, the printing plates are mounted on a support cylinderof a printing machine, especially by means of double-sided adhesives.

It has already become obviously necessary that the compressibility of aprinting plate must be adjusted, in particular to control the pressurein the nip, or in other words the pinch zone between two cylinders.Adjustment of the compressibility also makes it possible to increaseproductivity and to improve printing quality, especially in four-colorprinting.

The compressibility can be adjusted by means of a double-sided adhesiveused for fixation of the plate on the support cylinder, thisdouble-sided adhesive being itself compressible.

Unfortunately, this type of adhesive is cumbersome. In addition, it hasa serious adverse effect on the printer's budget and necessitatescomplex and successive mounting operations.

Another solution known in the prior art comprises placing a compressiblelayer between the photopolymer layer and the support.

For example, U.S. Pat. No. 4,582,777 describes a compressible platewhich, starting at a support film intended to be mounted on a printingcylinder, is provided with an adhesive, a compressible layer, anotheradhesive and a photopolymer layer.

Production of this printing plate is a complex process, since itnecessitates the presence of an adhesive between two successive layers.Thus the production costs are high.

European Patent 264894 describes an intermediate layer which acts as abarrier between a photopolymer layer and an intermediate elastomer layerdisposed between the photopolymer layer and the support layer. Thisintermediate layer can be compressible.

The barrier layer ensures that solvents used to develop the photopolymerlayer cannot penetrate into the intermediate layer. Nevertheless, itspresence complicates the production of the compressible plate and alsoadversely impacts its cost.

In addition, none of these compressible printing plates has found anindustrial application, which tends to show that they have not provensatisfactory.

The primary object of the invention is to alleviate these disadvantagesby proposing, for flexographic printing, a compressible plate which doesnot necessitate the use of compressible double-sided adhesives and whichcan be made simply, while achieving high printing quality.

In addition, the printers must sometimes deal with contradictoryrequirements in the same printing plate.

For example, a heavy printing pressure facilitates copious ink transfer,with “levelled” printing. This heavy printing pressure improves theprint quality of full tones.

On the other hand, light printing pressure makes it possible to optimizeprinting quality in four-color printing. This relatively light printingpressure is generally obtained by a soft compressible layer. It ensuresthat printing of faithful tone and better relief can be achieved, withlittle over-inking in the screened zones.

A compromise cannot always be achieved, to the point that two printingplates are often necessary to print the same color for a given subject.Thus a printer is often forced to use a plate mounted on a hard supportto print texts and full tones and a second printing plate of identicalnature but with a complementary image, mounted on a soft compressibleassembly, in order to print half-tones.

Another objective of the invention is to obviate the use of these twoplates and to provide a printing plate whose compressibility is notuniform over the entire printing plate, this plate thus being able toachieve optimal printing quality for both full tones and four-colorprinting.

Finally, the printing plates known in the prior art are delivered in theform of sheets of given size, typically 1016 mm×1524 mm, or else 1321mm×2032 mm.

This configuration as sheets is necessitated by the presence of a rigidsupport layer on one side of the plate and of an equally rigidprotective film on the other side of the plate, or in other words on thephotosensitive layer.

This configuration results in large losses, of at least 10%, when theprinters trim the plates to printing size before mounting them on thesupport cylinder of a printing machine.

These losses adversely impact the cost of printing plates even more.

Another object of the invention is to break away from these sizes inorder to limit the losses during trimming of the printing plates.

The primary object of the invention is therefore a compressibleflexographic printing plate, intended to be positioned on a supportcylinder of a printing machine, this plate comprising a photopolymerlayer and a compressible layer of a material containing voids formingbetween 10 and 60% of the volume of this compressible material, thematerial including at least 70% by weight of a polymer or of a polymermixture that is chemically compatible with the photopolymer.

Throughout the description, it will be understood that a given polymeris chemically compatible with another polymer when their solubilityparameters are similar and in particular when the difference thereof isless than or equal to 1.

The concept of solubility parameter is well defined for the personskilled in the art. It is defined in, for example, “Polymer Handbook”,by J. Brandrup and E. M. Immergut, 1975, Wiley IntersciencePublications, John Wiley & Sons, particularly in Chapter IV, pp. 337 ff.

For most of these polymers, the value of the solubility parameter can bedetermined directly or indirectly by means of solvents.

Certain polymers have several different phases, each having a specificsolubility parameter. In particular, this is the case of SEBS, which hasa rigid styrene phase and an elastomeric phase based on isobutylene andethylene.

For this type of polymer, the solubility parameter of the phaserepresenting the largest volume will be used for comparison with thevalue of the solubility parameter of another polymer, in order todetermine whether or not these two polymers are compatible.

In practice, chemically similar polymers are compatible. This is thecase, for example, of SIS with SBS or SEBS, and of a PU (polyurethane)polyester with another PU of polyester type.

In addition, the bond between the photopolymer layer and thecompressible layer can be established chemically, without the need toprovide an adhesive layer.

In fact, the compatible polymers of the compressible layer and of thephotopolymer layer will have a tendency to create an interdiffusion zoneat the interface, thus achieving strong adhesion.

The compressible plate according to the invention is therefore simplerin its production and necessarily less costly that the compressibleplates known in the prior art, since it does not necessitate a bondinglayer.

In addition, through appropriate choice of the materials used, it isalso possible to prevent migration of monomers from one layer to theother. This also ensures that the presence of a barrier layer is notrequired and that long-term stability of the structure and compositionof each of the two layers is achieved when they are in contact.

Thus the monomers contained in the photopolymer layer, and especiallythe cross-linking agents such as the acrylates, will not migrate towardthe compressible layer if their solubility parameter is closer to thatof the photopolymer than that of the polymers of the compressible layer.The same is true of other ingredients of the formulation, including thephotoinitiators.

Conversely, if the compressible layer contains monomers that, as will beseen hereinafter, are activated after production of the printing plate,these monomers do not migrate toward the photopolymer layer if theirsolubility parameter is closer to that of the polymers of thecompressible layer than that of the photopolymer.

In an advantageous embodiment of the printing plate according to theinvention, the voids present in the compressible layer are composed ofexpanded microspheres. These microspheres then represent between 1 and10% by weight of the material of which the compressible layer iscomposed.

In addition, the compressible layer contains between 0.1 and 3% byweight of protective agents, chosen in particular from among theantioxidants, the non-staining ozone-inhibitors of the phenolic type orthe waxes, used individually or as a mixture.

Preferably the compressible layer comprises a base polymer chosen fromone of the following groups:

-   -   elastomers of the styrene copolymer type, such as simple and        modified styrene-isoprene-styrene copolymer (SIS),        styrene-butadiene-styrene copolymer (SBS), styrene-diene-styrene        copolymer (SDS), styrene-branched butadiene copolymer ((S-B)x),        styrene-ethylene, butylene-styrene copolymer (SEBS),        styrene-ethylene, propylene-styrene copolymer (SEPS); these        elastomers being mixed as the case may be with double-block        copolymers containing, for example, vinyl groups, grafted chains        of isoprene, silanes or anhydrides;    -   thermoplastic elastomers obtained by dynamic vulcanization, such        as the mixtures of polypropylene/ethylene-propylene-diene        terpolymer type (PP/EPDM) or propylene/butadiene-acrylonitrile        copolymer type (PP/NBR);    -   pure or modified thermoplastic polyurethanes or even such        polyurethanes cross-linked with the isocyanates, examples being        thermoplastic polyurethane (TPU) or TPU modified with silane        functionalities;    -   elastomers of acrylonitrile type, such as        butadiene-acrylonitrile copolymer (NBR), carboxylated        butadiene-acrylonitrile copolymer (XNBR), hydrogenated        butadiene-acrylonitrile copolymer (HNBR) and elastomers of        functionalized butadiene-acrylonitrile copolymer type (NBR).

The compressible layer can also be provided with, in addition to thebase polymer, another polymer or polymeric or elastomeric additives thatare chosen from one or more of the following groups:

-   -   functionalized polyolefins, such as ethylene copolymers with        vinyl acetate (EVA), with vinyl alcohol (EVOH) or acrylate ester        (ethoxylated bisphenol A diacrylate (EBA), ethyl methacrylate        (EMA));    -   terpolymers of ethylene, vinyl acetate or acrylate ester with        maleic anhydride or glycidyl methacrylate,    -   copolymers of ethylene and octene, of propylene and hexene or        decene,    -   amide polyester or polyether,    -   elastomers of the ethylene-propylene-diene terpolymer type        (EPDM), butadiene-acrylonitrile copolymer type (NBR) or        hydrogenated butadiene-acrylonitrile copolymer type (HNBR).

This or these other polymer(s) or polymeric and elastomeric additive(s)makes or make it possible to adjust the mechanical properties of thecompressible layer.

The compressible layer of the printing plate according to the inventioncan also contain coagents, chosen in particular from among themonofunctional and multifunctional acrylates and methacrylates, such ashydroxypropyl methacrylate (HPMA), trimethylol propane triacrylate(TMPTA), trimethylolpropane trimethyl acrylate (TMPTMA), hexanediol1,6-diacrylate (HDDA), or glycerol 1,3-dimethacrylate (GDMA), alone orin combination.

The coagents are radical cross-linking promoters, and can be used toenhance the cross-linkability of the compressible layer.

The compressible layer can also contain functional polymers chosen inparticular from among the liquid polymers, examples beingethylene-propylene-diene terpolymer (EPDM), 1,2-polybutadiene (PB),hydroxy or epoxy poly(ethylene/butylene), or functionalizedbutadiene-acrylonitrile copolymer (NBR).

These liquid polymers are polymeric plasticizers.

The definitions of the compounds will be the same throughout thedescription, even if they are merely designated by their abbreviationshereinafter.

The compressible layer of the flexographic printing plate according tothe invention can also contain mineral fillers, especially such as pureor surface-modified silica, silicates, carbonates or stearates.

By means of these fillers it is possible to adjust the mechanicalproperties of the compressible layer.

Coloring agents can also be provided in the compressible layer,especially pigments and organic coloring agents, used in pure form or asmixtures.

By means of the coloring agents it is possible to adjust the capacity ofthe compressible layer to absorb UV light.

UV absorbers could also be incorporated in the formulation of thecompressible layer.

Preferably, the constituents of the compressible layer are present inthe following proportions by weight:

base polymer: from 35 to 90% other polymer or polymeric or elastomericadditives: from 0 to 60% coagents and functional polymers: from 0 to 20%fillers: from 0 to 10% microspheres: from 1 to 10% coloring agents: from0 to 1% UV absorbers: from 0 to 0.5% protective agents: from 0.1 to 3%

In a preferred embodiment, the compressible layer of the flexographicprinting plate according to the invention is provided with at least onefunctional monomer that has not yet reacted with the other polymer-typeconstituents of the compressible layer during production of the plate,and that is capable of being activated at a later time under the effectof a defined influence, to create or destroy other bonds between thepolymer chains.

If it is desired that the compressible plate contain several differentsystems capable of being activated after production of the plate inorder to create or destroy bonds between the polymer chains, it isadvisable to provide as many functional monomers or mixtures offunctional monomers as there are such systems.

By virtue of the presence of these functional monomers, it is possibleto modify the compressibility of the flexographic printing plate afterproduction thereof. As a result, zones of different compressibility,distributed in such a way that the plate is endowed with printingquality for both full tones and half-tones, can be obtained in theinterior of the same printing plate.

Preferably the functional monomer(s) is or are chosen from among themonofunctional and multifunctional acrylates and methacrylates,especially such as hydroxypropyl methacrylate (HPMA), trimethylolpropane triacrylate (TMPTA), trimethylolpropane trimethyl acrylate(TMPTMA), hexanediol 1,6-diacrylate (HDDA), or glycerol1,3-dimethacrylate (GDMA), alone or in combination.

These monomers are activated by UV light.

Advantageously there is mixed in the compressible layer containing thisor these functional monomer(s) at least one photoactivator orphotoinitiator, especially one that can be used to develop aphotosensitive layer and that preferably is temperature-resistant. Thisphotoactivator or photoinitiator can in particular be of the KIP 150 orEsacure® KTO46 type.

The presence of this photoactivator or photoinitiator promotes thereaction of the monomers in the presence of UV light.

This or these functional monomer(s) advantageously represent(s) between2 and 10% by weight of the compressible layer, while this or thesephotoactivator(s) or photoinitiator(s) advantageously represent(s)between 0 and 5% by weight of the compressible layer.

Thus the compressible layer of the printing plate according to theinvention can advantageously contain a functional monomer which makes itpossible to create, under UV irradiation, which is applied, for example,during development of the photosensitive layer of the printing plate,additional chains between the polymer-type constituents of thecompressible layer.

The printing plate according to the invention can also include a baselayer between the photopolymer layer and the compressible layer, thisbase layer being made of a material that is chemically compatible withthe polymers of the photopolymer layer and of the compressible layer.

Advantageously the flexographic printing plate according to theinvention is provided with a stabilization layer between thephotopolymer layer and the compressible layer.

This stabilization layer is configured, for example, in the form of anelastomeric layer, chemically compatible with the photopolymer and withthe polymer(s) of the compressible layer, provided with woven ornonwoven fibrous reinforcement.

The function of this layer is to limit the deformation of the image inthe circumferential direction during printing, in the contact zonesubjected to pressure. This layer offers a better guarantee thatregister will be maintained and that the images will be reproduced morefaithfully.

Another function of this stabilization layer is to guarantee sufficientdimensional stability of the compressible plate in its plane during themanipulations before it is positioned on a cylinder or sleeve,especially in the specific case in which the plate is not provided witha rigid support layer, as will be described hereinafter.

Advantageously, the flexographic printing plate according to theinvention can also contain a contrast layer at the surface of thephotopolymer layer.

Such a contrast layer is described in particular in European Patent0456336, which can be consulted for particulars.

The flexographic printing plate according to the invention can alsocontain, at the surface of the photopolymer layer, a layer on which animage can be formed on the photopolymer layer by digital means.

Such a layer is described in particular in International PatentApplication PCT 97/25206, which can be consulted.

The flexographic printing plate according to the invention can beconfigured in the form of a sheet of defined size or else in the form ofa roll.

The configuration in the form of a roll is made possible by the factthat the printing plate according to the invention does not have to beprovided with a rigid support layer or protective sheet, in contrast tothe plates known in the prior art.

This configuration in the form of a roll allows the user to trim theprinting plates to the desired size before development of thephotosensitive layer to generate an image.

The configuration in the form of a roll makes it possible to limit thetrimming losses compared with a configuration in fixed sizes.

When the printing plate according to the invention is configured in theform of a sheet, it can be advantageously provided on its inner facewith a film of plastic or metal material, which ensures betterdimensional stability of the plate while it is being manipulated beforebeing positioned on a cylinder.

Regardless of whether the plate is configured as a sheet or as a roll,it can be advantageously provided on its inner face with a self-sealingadhesive. This allows the compressible plate to be attached to a supportfilm, which itself is mounted on the printing machine or on a printingsleeve such as that described in European Patent 0683040, which can beconsulted.

This self-sealing adhesive is provided on the film of plastic or metalmaterial when the printing plate contains such a film.

The presence of a self-sealing adhesive on the inner face of thecompressible plate simplifies the mounting thereof and limits thequantity of consumable products to be purchased and used by the printer.

The invention also relates to a flexographic printing plate such asdescribed hereinabove wherein the photopolymer layer has been developed.

The invention also relates to a process for making a flexographicprinting plate comprising a photopolymer layer, this process having thefollowing stages:

-   -   preparation of a mixture containing at least 70% by weight of        one or more polymers that are chemically compatible with the        photopolymer and 1 to 10% by weight of microspheres in the        expanded or non-expanded state or of mixtures of two types of        macrospheres or of blowing agents;    -   extrusion of this mixture, during which the mixture expands to        form a compressible layer containing a multiplicity of voids;    -   extrusion of the photopolymer to form the photopolymer layer;    -   rolling of the photopolymer layer against the previously formed        compressible layer, the pressure of one layer against the other        making them adhere to one another; and    -   activation of a system to create, in the compressible layer,        long interlinked polymer chains, wherein this activation stage        can be performed before, during or after rolling of the        photopolymer layer against the compressible layer.

In a first embodiment of the process, the system for creatinginterlinked chains is a cross-linking system, which is activated, forexample, by heat, by isocyanates, by electron bombardment or else byX-rays.

In another embodiment of the process, this system for creatinginterlinked chains comprises anchoring the polymer chains by theintermediary of a crystal phase at working temperature or at roomtemperature.

Advantageously, the cross-linking of at least one part of the mixtureconstituting the compressible layer is performed during extrusion of themixture, in the case of a system with cross-linking of thermal type.

The process according to the invention can also have a stage ofcorrecting the size of the compressible layer or of straightening itafter it has been extruded, to guarantee constant nominal thickness.

Preferably the compressible layer has a thickness of between 0.5 and 4.0and especially of between 0.5 and 1.5 mm.

The process according to the invention is simple to use compared withthe known production processes, and it permits a productivity gain ashigh as 30%.

The mixture advantageously contains between 0.1 and 3% by weight ofprotective agents, chosen in particular from among the antioxidants, thenon-staining ozone-inhibitors of the phenolic type or the waxes, usedindividually or as a mixture.

Preferably the mixture comprises a base polymer chosen from one of thefollowing groups:

-   -   elastomers of the styrene copolymer type, such as simple and        modified SIS, SBS, SDS, (S-B)x, SEBS, SEPS as well as mixtures        of such elastomers with double-block copolymers containing vinyl        groups, grafted chains of isoprene, silanes or anhydrides;    -   thermoplastic elastomers obtained by dynamic vulcanization, such        as, for example, the mixtures of PP/EPDM or PP/NBR type;    -   pure or modified thermoplastic polyurethanes or even such        polyurethanes cross-linked with the isocyanates, examples being        TPU or TPU modified with silane functionalities;    -   elastomers of the acrylonitrile type, such as NBR, XNBR, HNBR        and functionalized NBR elastomers.

The mixture can also be provided with, in addition to the base polymer,another polymer or polymeric or elastomeric additives that are chosenfrom one or more of the following groups:

-   -   functionalized polyolefins, such as ethylene copolymers with        vinyl acetate (EVA), with vinyl alcohol (EVOH) or acrylate ester        (EBA, EMA);    -   terpolymers of ethylene, vinyl acetate or acrylate ester with        maleic anhydride or glycidyl methacrylate,    -   copolymers of ethylene and octene, or of propylene and hexene or        decene,    -   amide polyester or polyether,    -   elastomers of the EPDM, NBR or HNBR type.

The mixture of which the compressible layer is composed can also containcoagents, chosen in particular from among the monofunctional andmultifunctional acrylates and methacrylates, such as HPMA, TMPTA,TMPTMA, HDDA, or GDMA, alone or in combination.

The mixture can also contain functional polymers chosen in particularfrom among the liquid polymers, examples being EPDM, 1,2-PB, hydroxy orepoxy poly(ethylene/butylene), or functionalized NBR.

This mixture can also contain mineral fillers, especially such as pureor surface-modified silica, silicates, carbonates or stearates.

Coloring agents can also be provided in the mixture, especially pigmentsand organic coloring agents, used in pure form or as mixtures.

The different constituents of the mixture that permit formation of thecompressible layer are advantageously present in the followingproportions by weight:

base polymers: 35 to 90%  other elastomer or polymeric or elastomericadditives: 0 to 60% coagents and functional polymers: 0 to 20% fillers:0 to 10% microspheres or blowing agents: 1 to 10% coloring agents: 0 to1%  UV absorbers:  0 to 0.5% protective agents: 0.1 to 3%  

By virtue of the presence of microspheres or of chemical blowing agentsin a proportion of 1 to 10% by weight of the mixture, it is possible toobtain in the extruded compressible layer a void ratio of 10 to 60% byvolume. This ratio corresponds to the volume fraction of gas present inthe compressible layer after production.

Preferably the compressible layer has closed cells, which are moreresistant mechanically and chemically.

The microspheres contain a solvent which, under the effect of heat ataround 100 to 150° C., causes expansion of the microspheres.

The blowing agents in turn have the effect of chemically expanding thecompressible layer during the hardening phase.

The process according to the invention also comprises providing, in themixture with which the compressible layer is formed, at least onefunctional monomer that does not react with the other constituents ofthe polymer type of the compressible layer and which can be activated ata later time under the effect of a defined influence.

Preferably the functional monomer(s) is or are chosen from themonofunctional and multifunctional acrylates and methacrylates,especially such as HPMA, TMPTA, TMPTMA, HDDA, or GDMA, alone or incombination.

This or these functional monomer(s) is or are advantageously mixed withat least one photoactivator or photoinitiator, especially one that canbe used to develop a photosensitive layer.

When the process comprises providing such monomers in the mixture, itadvantageously includes an additional stage in which this or thesefunctional monomer(s) is or are activated in order to modify thecompressibility of the printing plate selectively.

In the compressible plate obtained by the process according to theinvention, the photopolymer layer advantageously has a thickness ofbetween 0.4 and 2.5 mm.

The process according to the invention can also include another stagecomprising forming a base layer between the photopolymer layer and thecompressible layer.

The base layer preferably has a thickness of between 0.1 and 2 mm.

It can also include an additional stage for formation of a stabilizationlayer between the photopolymer layer and the compressible layer, thislayer being configured in the form of an elastomeric layer reinforced bya fibrous reinforcement, this elastomer being chemically compatible withthe polymers of the photopolymer layer and of the compressible layer.

This stabilization layer preferably has a thickness of between 0.2 and0.5 mm.

The invention will be better understood and other objectives, advantagesand characteristics thereof will become clearer upon reading thedescription provided hereinafter of non-limitative practical examples ofthe invention with reference to the attached figures, wherein:

FIG. 1 is a schematic view in section of a compressible plate accordingto the invention, comprising a photopolymer layer and a compressiblelayer;

FIG. 2 is a schematic view in section of a compressible plate accordingto the invention, including a base layer between the compressible layerand the photopolymer layer;

FIG. 3 is a schematic view in section of a compressible plate accordingto the invention, provided with a stabilization layer between thephotopolymer layer and the compressible layer;

FIG. 4 is a schematic view in section of a compressible plate accordingto the invention, comprising a photopolymer layer, a compressible layerand a support film;

FIG. 5 shows two curves of the plane compression between parallelplatens of a compressible plate according to the invention (in microns)as a function of the pressure exerted on the plate (in kg/cm²).

In FIGS. 1 to 4, the photopolymer layer is schematically illustrated inundeveloped form on the right side of these figures and in developedform on the left side of these figures.

A compressible plate is generally delivered to the photoengravers withan undeveloped photosensitive layer, as illustrated on the right side ofthese figures. It can also be delivered directly to a printer, afterdevelopment of the photopolymer layer (as illustrated on the left sideof these figures).

Referring now to FIG. 1, there is illustrated the basic version of thecompressible printing plate according to the invention.

This plate 1 is provided with a photopolymer layer 10 and a compressiblelayer 11.

In the example illustrated in FIG. 1, this compressible layer 11 is madewith SEBS as its base, while photopolymer layer 10 is made with SIS.

This compressible layer 11 is made from a mixture comprising 76% byweight of SEBS, 16% by weight of a polymer of functionalized olefintype, 5% by weight of multi-acrylates, 2% by weight of expandablemicrospheres of Expancell® type (registered trademark of the AKZO NOBELCo.) and of the DU type, and 1% by weight of protective agents of thephenolic type, such as Irganox® 1010.

This mixture is first extruded and voids are created in the interior ofthe mixture by virtue of the microspheres, which expand during extrusionunder the effect of the solvent that they contain.

The elastomer layer is then cross-linked by electron bombardment with adose of 25 to 250 kGy to obtain optimal mechanical stability and alsoresistance to chemical expansion.

The efficacy of electron bombardment is enhanced by the fact thatmulti-acrylates are present in the mixture.

Consequently, compressible layer 11 is not affected by solvent attackduring development of the image of photopolymer layer 10 and duringflexographic printing.

The compressible layer obtained has a modulus of elasticity, measured inplane compression between two parallel platens, on the order of 6 MPa.

In general, the modulus of elasticity of a compressible layer for aprinting plate according to the invention ranges between 1 and 25 MPaand preferably between 2 and 15 MPa.

This photopolymer layer is therefore made with a base of SIS and aphotoinitiator as described in European Patent 0456336. Thus this layeris made starting from a mixture comprising approximately: 85% by weightof SIS, 4% by weight of HDDA, 3% by weight of HDDMA, 1.4% by weight of aphotoinitiator of the Irgacure 651® type, 1.4% by weight of2,6-di-t-butyl-p-cresol, 0.2% by weight of calcium stearate, 0.64% byweight of an ozone-inhibiting paraffin wax, 4.3% by weight of adepolymerized polyisoprene, 0.02% by weight of Tinurin 1130® and 0.04%by weight of an antioxidant of the Irganox 1010® type of the Ciba GeigyCo.

This mixture is extruded and then applied with a certain pressureagainst previously formed compressible layer 11.

This pressure causes adhesion between the two layers 10 and 11, becauseof the fact that the two materials are chemically compatible.

In fact, for this photopolymer layer 10 and this compressible layer 11,the monomers having the solubility parameters listed below can beconsidered:

Section of polymer Value of solubility under consideration parameterUnit Polyisoprene (in SIS polymer) 8.2 (cal/cm³)^(1/2) Polyisobutylene(in SEBS)  7.85 (cal/cm³)^(1/2) Polyethylene (in SEBS) 7.7(cal/cm³)^(1/2) Polystyrene (in SIS) 9.1 (cal/cm³)^(1/2) (in SEBS) 8.5to 10.6 Acrylates and methacrylates 8.3 to 8.9  (cal/cm³)^(1/2)

The solubility parameter of the SIS used here is about 8.2. In fact,this particular SIS has low styrene content and it exhibitssubstantially the same solubility parameter as the polyisoprene section.

In addition, the solubility parameter of the elastomeric phasecorresponding to the polyisobutylene and polyethylene sections, whichare present in substantially equal quantities in the SEBS underconsideration, are used for the SEBS. The solubility parameter of theSEBS is therefore around 7.8.

Thus the difference between the two solubility parameters is much lessthan 1, and so the SIS and the SEBS are compatible.

In addition, this table reveals that the values of the solubilityparameters of the acrylates and methacrylates are much closer to thesolubility parameter of the polyisoprene section contained in the SIS ofthe photopolymer layer than to the solubility parameter of thepolyethylene section in the SEBS of the compressible layer.

Consequently, the acrylates contained in the photopolymer layer do nottend to migrate toward the compressible layer.

It is also known that SIS and SEBS are chemically similar.

For example, the polystyrene blocks or segments are soluble in oneanother, the only limitation being the influence of the molecular weightof the segments. Furthermore, the solubility parameter of polyisoprenediffers from that of polyethylene or from that of polyisobutylene merelyby a value smaller than 0.5.

This chemical compatibility between the polymers used in the two layersimmediately assures excellent adhesion and good continuity between thetwo layers 10 and 11.

In addition, migration or diffusion of unreacted monomers from layer 10to layer 11 or vice versa cannot occur, specifically for thermodynamicreasons.

Because of this absence of migration, these monomers cannot alter, inthe course of time, one or the other of these two layers bonded to oneanother.

This has been verified experimentally by an accelerated aging test.

It was performed in an oven at 45° C. with graduated durations of 7, 14,21 and 28 days, in a direct comparison between the plate according tothe invention and a reference plate of EPIC type (commercial product ofPolyfibron). The measurements of hardness and of plane compressionbetween platens performed after each aging period did not reveal anynoteworthy evolution in one direction or the other for the two plates.

Compressible plate 1 according to the invention is stable, and there isno need to provide a support layer on the back, or in other words on theside of the compressible layer opposite the photopolymer layer, thissupport layer generally being composed, in the prior art, of a polyesterfilm.

Thus the compressible plate according to the invention can be configuredjust as well in the form of a sheet as of a roll. As indicatedhereinabove, this configuration as a roll represents a gain on the orderof 10% in trimming efficiency.

Thus the printing plate according to the invention can be fixed directlyon a cylinder of a printing machine by the intermediary of adouble-sided adhesive on its back.

The printing plate is developed by the usual technique. This step isadvantageously performed after the plate has been fixed on a supportfilm or on a sleeve.

Referring now to FIG. 2, printing plate 2 according to the invention canalso be provided with a base layer 22 between photopolymer layer 20 andcompressible layer 21.

For reasons of simplicity, the base layer can be made of the samephotopolymeric material as photopolymer layer 20 containing the image.It can also be made of a different material, provided it is chemicallycompatible with the polymers of photopolymer layer 20 and ofcompressible layer 21.

This base layer is preferably formed by irradiation of the back of thephotopolymer layer before or after extrusion of the compressible layer.

This exposure of the back is made possible by the fact that compressiblelayer 21 is transparent to UV radiation. As indicated above, thepresence of coloring agents in compressible layer 21 makes it possibleto adjust the ability of the compressible layer to absorb or transmit UVlight.

Referring now to FIG. 3, there is illustrated another compressibleprinting plate according to the invention. This plate 3 is provided witha stabilization layer 32 between photopolymer layer 30 and compressiblelayer 31.

This stabilization layer 32 is made from an elastomer and fibers, and ispreferably configured in the form of a reinforcement of nonwoven orwoven type.

This fibrous reinforcement 320 is preferably of open nature, in order topermit good impregnation of this reinforcement by the polymer of thestabilization layer.

The modulus of elasticity of the stabilization layer in the nip rangesbetween 50 and 2000 MPa, preferably between 200 and 1000 MPa, regardlessof direction in the plane of the compressible plate.

If compressible layer 31 is based on SEBS, in common with layer 11 ofplate 1 illustrated in FIG. 1, the polymer of stabilization layer 32will also be made advantageously of SEBS. Nevertheless, a differentpolymer or mixture of polymers could be used provided it is chemicallycompatible with the polymers of photopolymer layer 30 and ofcompressible layer 31.

Preferably the polymer of this stabilization layer 32 will have apredefined refractive index for UV light. This index will be chosen tofacilitate transmission of the incident energy between layers 30 and 31.

It will also be possible to add supplementary layers with refractiveindices for light such that they enhance the transmission of theincident energy. UV absorbers can also be incorporated in thesesupplementary layers.

The function of these UV absorbers is to reduce or eliminate reflectionof the UV radiation at the interface between the photopolymer layers andthe compressible layer. This reflection can cause parasiticcross-linking and clogging of the reserves (zones that typically are notcross-linked in a printing full tone) in the case of small thicknessesof the photopolymer layer.

Referring now to FIG. 4, there is illustrated a compressible layer 4comprising successively a photopolymer layer 40, a compressible layer 41and a support film 42.

This film 42 can be made of plastic or metal material. It ensures betterdimensional stability of the plate before this is positioned on acylinder.

This film is provided more particularly on plates configured in the formof sheets.

There will now be described two practical examples of a compressibleplate according to the invention, provided with a compressible layerwhose compressibility can be modulated or adjusted after production ofthe compressible plate.

EXAMPLE 1

In a first practical example of such a plate, this is provided with acompressible layer of expanded polyurethane.

This compressible layer is obtained from a prepolymer with isocyanateterminal groups and a triol, with a chain-lengthening agent, asurfactant and a catalyst being added to the mixture. In this mixturethere are incorporated monomers of the pentaerythritol tetraacrylatetype and a photoinitiator of Irgacure 651® type.

The proportions by weight of these constituents in the mixture are asfollows: about 85% of polyol, about 8% of triol, about 1% ofchain-lengthening agent with terminal isocyanate groups, about 4% oftetraacrylate and about 2% of photoinitiator.

During extrusion of this mixture, it is expanded by addition water in aproportion of 0.1 parts by weight of polyol. The primary system forcreating interlinked polymer chains is a cross-linking system ofisocyanate type.

A layer of polyurethane-base photopolymer is then applied, this layerbeing made to adhere to the compressible layer, or in other words anirreversible bond being established between the two layers, byinterpenetration of surface polymer chains.

Thus, in the compressible plate obtained, the cross-linking system ofthe isocyanate type has already been activated to form the polyurethane.

In contrast, the compressible layer contains monomers of thepentaerythritol tetraacrylate type that have not yet reacted with theother constituents of the compressible layer.

The printing plate obtained can be irradiated at a later time with UVradiation in order to initiate cross-linking of the tetraacrylate and toharden the compressible layer.

In this case, therefore, the monomers of the pentaerythritoltetraacrylate comprise, for creation of other bonds between the polymerchains, a system that is still latent after production of the plate andthat can be activated at a later time.

In this particular example, the UV radiation step can be accomplished inparticular during the development of the photopolymer layer of theprinting plate.

Thus the cross-linking that takes place in the compressible layer duringproduction of the printing plate makes it possible to obtain acompressible layer that is soft but chemically resistant. Thiscompressible layer ensures that the photosensitive layer can bedeveloped and that four-color printing in half-tones can be optimized.

The complementary cross-linking system due to the presence of monomersof pentaerythritol tetraacrylate type makes it possible to harden thecompressible layer during, in particular, irradiation of the printingplate. The parts of the compressible layer that become harder ensurethat full tones of great quality can be printed.

When UV irradiation is used to harden the compressible layerselectively, this selective hardening is achieved without asupplementary operation, which is a noteworthy advantage for thephotoengravers.

Referring now to FIG. 5, there are illustrated two curves of thecompression (in microns) of the printing plate just described in theforegoing example as a function of the pressure exerted (in kg/cm²). Itis noted that the ratio between the tangent moduli of the two layers SIand AI ranges from 1 to 1.75.

Curve SI (^(———)) corresponds to tests performed with the printing plateafter it has been produced and before it has been subjected to UVirradiation.

Curve AI ( - - - ) corresponds to tests performed with this same plate,but in zones that have been subjected to UV irradiation.

These two curves AI and SI demonstrate that the printing plate has beenhardened to a noteworthy extent in the zones subjected to UVirradiation.

Of course, the irradiation with UV light can be modulated in durationand energy, thus ensuring that the hardness imparted to the compressiblelayer can also be modulated.

In the example mentioned hereinabove, the compressible plate is hardenedin a spatial distribution corresponding to the image to be printed.

Nevertheless, it would be possible to choose any other spatialdistribution that would be favorable for printing. The printing platecould also be subjected to uniform irradiation, in order to harden theplate uniformly.

In practice, the photoengraver or the printer therefore has theopportunity to choose the compressibility of the printing plate to matchhis needs.

EXAMPLE 2

Another example of a printing plate according to the invention, whereinthe compressibility can be modulated, will now be described.

This printing plate is provided with a compressible layer of SEBS.

Thus this compressible layer is obtained by extrusion of a mixturecontaining SEBS, expanded and non-expanded microspheres of theExpancell® type, functional monomers of the pentaerythritoltetraacrylate type and one or more photoinitiators of the Irgacure 651®type.

These functional monomers represent between 2 and 10% by weight of themixture and preferably about 5% by weight, while the other constituentsare present in the mixture in the following proportions by weight:

-   -   SEBS, from 46 to 94% and preferably between 61.95 and 83.65%;    -   coloring agents from 0.01 to 1% and preferably 0.05%;    -   microspheres from 1 to 10% and preferably 3 to 5%;    -   polymeric additives from 3 to 15%;    -   protective agents from 0.1 to 3%;    -   photoactivators from 0.2 to 5%;    -   fillers from 0 to 10% and preferably 5%.

The compressible layer is cross-linked by electron bombardment with adose of about 25 to 250 kGy during production of the compressible plate.

The functional monomers in the compressible plate produced in this wayhave not yet been activated, but they can be activated by means ofsubsequent UV exposure in such a way that the compressible layer ishardened selectively, as already described for the first examplehereinabove.

It would also be possible to envision other examples of a compressibleplate, containing different functional monomers that can be activated bydifferent influences, in order that the compressibility of the plate canbe modulated even more.

The functional monomers provided in the compressible layer can lead tothe creation of other bonds between the polymer chains, as shown by thetwo examples illustrated hereinabove.

There can also be provided monomers that, when they are activated,destroy polymer chains.

In this case the compressible layer obtained during production of thecompressible plate will be relatively hard, but this layer can be mademore flexible at a later time when these monomers are activated.

In general, a printing plate whose compressibility can be modulated isamenable to better standardization of its thickness.

Above all, the modulation of the compressibility in the plate makes itpossible to obviate the use of different, more or less compressibleself-sealing adhesives, of two plates of different thickness or even ofseveral different plates, as is the case in the prior art.

In general, a compressible, ready-to-use plate according to theinvention has a thickness of between 0.76 and 6.25 mm.

Preferably it has a thickness of between 1.0 and 2.8 mm.

It can also have a thickness which is standardized in the industry, andwhich can be 1.14 mm, 1.70 mm, 2.29 mm, 2.54 mm, 2.70 mm, 2.84 mm, 3.17mm, 3.94 mm or else 4.32 mm.

The compressible plate can also have a standardized thickness of 1.59mm, 2.15 mm or 2.74 mm. This thickness corresponds to that of astandardized plate increased by the thickness of a standard compressiblelayer.

Finally, the printing plate according to the invention preferably has astandardized thickness of 1.59 mm, 2.15 mm, 2.29 mm or 2.7 mm.

The flexographic printing plate according to the invention can be usedin the usual way.

In particular, it is selectively fixed on a sleeve or support filmbefore UV irradiation, specifically at the sites that contain the zonesto be printed.

As a result, time can be saved during the mounting process, the risks ofhuman errors can be avoided and perfect reference marking can also beguaranteed when the compressible plate is developed after the mountingprocess.

UV irradiation of the printing plate can be accomplished by standard ordigital means, after it has been mounted on a film or on a sleeve.

The reference symbols appended to the technical characteristicsspecified in the claims are provided merely to improve understandingthereof without limiting the scope thereof.

1. In a compressible printing plate, the improvements comprising aphotopolymer layer and a compressible layer of a material containingvoids forming between 10 and 60% of the volume of the material, thematerial including at least 70% by weight of a polymer or of a polymermixture that is chemically compatible with the photopolymer layer andthe material including at least one functional monomer that has not yetreacted with the polymer or polymer mixture of the material.
 2. Aprinting plate according to claim 1, wherein the voids present in thecompressible layer are composed of expanded microspheres.
 3. A printingplate according to claim 2, wherein the microspheres represent between 1and 10% by weight of the material of which the compressible layer iscomposed.
 4. A printing plate according to claim 1, wherein thecompressible layer contains between 0.1 and 3% by weight of protectiveagent antioxidants, non-staining ozone-inhibitors of the phenolic typeor waxes individually or as a mixture.
 5. A printing plate according toclaim 1, wherein the compressible layer comprises a base polymer of oneof the following groups: elastomers of the styrene copolymer type, suchas simple and modified SIS, SBS, SDS, (S-B)x, SEBS, SEPS; theseelastomers being mixed as the case may be with double-block copolymerscontaining, for example, vinyl groups, grafted chains of isoprene,silanes or anhydrides; thermoplastic elastomers obtained by dynamicvulcanization, such as the mixtures of PP/EPDM or PP/NBR type; pure ormodified thermoplastic polyurethanes or even such polyurethanescross-linked with the isocyanates, examples being TPU or TPU modifiedwith silane functionalities; or elastomers of the acrylonitrile type,such as NBR, XNBR, HNBR and functionalized NBR elastomers.
 6. A printingplate according to claim 5, wherein the compressible layer is alsoprovided with, in addition to the base polymer, another polymer orpolymeric or elastomeric additives of one or more of the followinggroups: functionalized polyolefins, such as ethylene copolymers withvinyl acetate (EVA), with vinyl alcohol (EVOH) or acrylate ester (EBA,EMA); terpolymers of ethylene, vinyl acetate or acrylate ester withmaleic anhydride or glycidyl methacrylate, copolymers of ethylene andoctene, of propylene and hexene or decene, amide polyester or polyether,or elastomers of the EPDM, NBR or HNBR type.
 7. A printing plateaccording to claim 5, wherein the compressible layer contains coagentsof monofunctional or multifunctional acrylates or methacrylates, HPMA,TMPTA, TMPTMA, HDDA, or GDMA, alone or in combination.
 8. A printingplate according to claim 5, wherein the compressible layer contains afunctional liquid polymer EPDM, 1,2-PB, hydroxy or epoxypoly(ethylenelbutylene), or functionalized NBR.
 9. A printing plateaccording to claim 5, wherein the compressible layer contains one ormore of mineral fillers, pure or surface-modified silica, silicates,carbonates or stearates.
 10. A printing plate according to claim 5,wherein the compressible layer contains coloring agents, pigments andorganic coloring agents, in pure form or as mixtures.
 11. A printingplate according to claim 5, wherein the constituents of the compressiblelayer are advantageously present in the following proportions by weight:base polymers: from 35 to 90% other polymers or polymeric or elastomericadditives: from 0 to 60% coagents and functional polymers: from 0 to 20%fillers: from 0 to 10% coloring agents: from 0 to 1% microspheres: from1 to 10% protective agents: from 0.1 to 3%.


12. A printing plate according to claim 1, wherein the functionalmonomer(s) is or are monofunctional and multifunctional acrylates andmethacrylates of the HPMA, TMPTA, TMPTMA, HDDA, or GDMA type, alone orin combination.
 13. A printing plate according to claim 1, wherein thereis mixed in the compressible layer at least one photoactivator orphotoinitiator.
 14. A printing plate according to claim 1, wherein thisor these functional monomer(s) represent(s) between 2 and 10% by weightof the compressible layer, while this or these photoactivator(s) orphotoinitiator(s) is or are advantageously present in proportions ofbetween 0 and 5% by weight of the compressible layer.
 15. A printingplate according to claim 1, provided with a base layer between thephotopolymer layer and the compressible layer, this base layer beingmade of a material that is chemically compatible with the polymers ofthe photopolymer layer and of the compressible layer.
 16. A printingplate according to claim 1, provided with a stabilization layer betweenthe photopolymer layer and the compressible layer.
 17. A printing plateaccording to claim 16, wherein the stabilization layer is configured inthe fonn of an elastomeric layer that is chemically compatible with thephotopolymer and with the polymer(s) of the compressible layer, andwhich is provided with woven or nonwoven fibrous reinforcement.
 18. Aprinting plate according to claim 1, also containing, at the surface ofthe photopolymer layer, a contrast layer.
 19. A printing plate accordingto claim 1, also containing, at the surface of the photopolymer layer, alayer on which an image can be formed on the photopolymer layer bydigital means.
 20. A printing plate according to claim 1, configured inthe form of a sheet and provided on one face with a film of plastic ormetal material.
 21. A printing plate according to claim 1, provided onone face with a self-sealing adhesive.
 22. A printing plate according toclaim 1, wherein the photopolymer layer has been developed.
 23. In acompressible flexographic printing plate to be positioned on a supportof a printing machine, the improvements comprising: a photopolymer layerfor printing; and a compressible layer positioned to be between thephotopolymer layer and the support when the compressible flexographicprinting plate is on the support, the compressible layer comprising amaterial containing between 10 and 60% of voids by volume, at least 70%by weight of at least one of a polymer or a polymer mixture that ischemically compatible with the photopolymer layer, and at least onefunctional monomer that has not yet reacted with the polymer or polymermixture for being activated in coordination with the photopolymer layerto create zones of the compressible layer having varyingcompressibility.
 24. In a compressible flexographic printing plate to bepositioned on a support of a printing machine, the improvementscomprising: a photopolymer layer for printing; and a compressible layerpositioned to be between the photopolymer layer and the support when thecompressible flexographic printing plate is on the support, thecompressible layer comprising a material containing between 10 and 60%of voids by volume, at least 70% by weight of at least one of a polymeror a polymer mixture that is chemically compatible with the photopolymerlayer and cross-linkable by a first activation means to have apredetermined compressibility and has reacted during the manufacture ofthe compressible layer, and at least one functional monomer of a typereacting under the effect of a second activation means different fromthe first activation means, said photopolymer layer being activatable bysaid second activation means to receive an image to be printed, saidmonomer having not reacted with the polymer or polymer mixture and isactivatable with the activation of the photopolymer layer by said secondactivation means to create zones of the compressible layer havingvarying compressibility corresponding to the image on the photopolymerlayer.
 25. Flexographic printing plate to be positioned on a support ofa printing machine, comprising: a photopolymer layer having beendeveloped by a predetermined activation means in view to have an imageto be printed with zones for printing full tones requiring heavyprinting pressure and zones for four-color printing requiring lightprinting pressure, positioned between the photopolymer layer and thesupport when the compressible flexographic printing plate is on asupport; and a compressible layer comprising a material containing (A)between 10 and 60% of voids by volume, at least 70% by weight of atleast one of a polymer or a polymer mixture that is cross-linked by apolymer activation means and is chemically compatible with thephotopolymer layer and (B) at least one functional monomer not reactablewith the polymer activation means during manufacture of the flexographicprinting plate before development of the photopolymer layer but to reactin response to the activation means causing the development of thephotopolymer layer, wherein the compressible layer is hardened in aspatial distribution corresponding to the image to be printed of thephotopolymer layer by having been activated by the polymer activationmeans at the development of the photopolymer layer in view to insureheavy printing pressure in the image zones for printing full tones andlight printing pressure in the image zones for four-color printing.